Intermittent operation
Some devices may transmit only at selected times or under particular conditions. A quiet spectrum at one instant is therefore not proof that a location is clear.
Yes, some surveillance can still be investigated when no obvious RF transmission is present — but it requires more than a simple RF detector.
A device that is not transmitting may not present an obvious RF signal at the moment it is examined. That is why professional TSCM should not be reduced to watching a handheld RF detector for an alarm.
Some devices may transmit only at selected times or under particular conditions. A quiet spectrum at one instant is therefore not proof that a location is clear.
A recorder can capture information locally without continuously radiating an RF signal. Physical and electronic inspection remains important.
Spectrum analysis, physical examination and other appropriate detection methods provide different pieces of evidence.